1. Field of the Invention
The invention relates to an air-fuel ratio control system and an air-fuel ratio control method for an internal combustion engine that control the air-fuel ratio of exhaust gas entering a catalyst.
2. Description of the Related Art
An air-fuel ratio control system for an internal combustion engine that controls the air-fuel ratio of exhaust gas entering a catalyst based on the output values of an air-fuel ratio sensor and an oxygen concentration sensor is described in, for example, Japanese Patent Application Publication No. 2005-113729 (JP-A-2005-113729). This air-fuel ratio control system has an air-fuel ratio sensor provided upstream of a catalyst in the exhaust passage of the internal combustion engine and an electromotive force type oxygen concentration sensor provided downstream of the catalyst. According to this air-fuel ratio control system, a feedback correction value is calculated by performing a proportional integral derivative processing (so-called PID processing) to the deviation between the output value of the oxygen concentration sensor and the target value of that output value (corresponding to “target air-fuel ratio”). This deviation will be referred to as “downstream-side deviation” where necessary. Then, feedback control is performed such that the difference between the air-fuel ratio obtained from the output value of the air-fuel ratio sensor corrected by the foregoing feedback correction value and the target air-fuel ratio is controlled to be zero so that the catalyst upstream-side air-fuel ratio equals the target air-fuel ratio.
In general, for example, a deviation (i.e., the variation of detection by the airflow meter) unavoidably arises between the intake air flowrate detected by an airflow meter, which is used to determine the amount of fuel to be injected from the injector, and the actual intake airflow rate, and a deviation (i.e., the variation of injection from the injector) unavoidably arises between the required fuel injection amount that the injector is instructed to inject and the amount of fuel actually injected. Such deviations will be collectively referred to as “error of fuel injection amount”. Further, the output value of a limiting-current type oxygen concentration sensor that is typically used as the foregoing air-fuel ratio sensor tends to include an error. Hereinafter, the error of fuel injection amount and the error of the upstream-side air-fuel ratio sensor will be collectively referred to as “intake/exhaust system error” where necessary.
The aforementioned feedback control value includes a value of an integral term, that is, a value obtained by multiplying an integral value of deviation, which is updated by integrating the downstream-side deviation, by a feedback gain. Therefore, even when the intake/exhaust system error occurs, it may be compensated for by the integral term by performing the foregoing feedback control, and therefore the air-fuel ratio may be made equal to the target air-fuel ratio. In other words, the value of the integral term (or the integral value of deviation) may indicate the magnitude of the intake/exhaust system error.
Many air-fuel ratio control systems of this kind perform an integral term learning process in which the value of the integral term (or the integral value of deviation) as mentioned above is recorded while the recorded value of the integral term (will be referred to also as “learning value of the integral term”) is repeatedly updated (learned) at given time intervals.
Meanwhile, the value of the integral term (or the learning value of the integral term) converges to the value that accurately represents the magnitude of the intake/exhaust system error (will be referred to as “target convergence value”). If the value of the integral term (or the learning value of the integral term) is equal to the target convergence value, it indicates that an actual air-fuel ratio that is being treated equally as the target air-fuel ratio (will be referred to as “control center air-fuel ratio”) by the air-fuel ratio control system, is actually equal to the target air-fuel ratio.
When the control center air-fuel ratio is equal to the target air-fuel ratio, the intake/exhaust system error may be properly compensated for, and thus the air-fuel ratio may be properly controlled to the target air-fuel ratio. Note that, in the case of the system described in JP-A-2005-113729, the fact that the control center air-fuel ratio is equal to the target air-fuel ratio indicates that the air-fuel ratio obtained from the output value of the air-fuel ratio sensor corrected by the feedback correction value is equal to the catalyst upstream-side air-fuel ratio.
On the other hand, when the value of the integral term (or the learning value of the integral term) is deviating from the target convergence value, the control center air-fuel ratio becomes a value deviating from the target air-fuel ratio. In this case, there is a possibility that the intake/exhaust system error is not properly compensated for, and thus the air-fuel ratio is not properly controlled to the target air-fuel ratio. Therefore, it is preferable to maintain the value of the integral term (or the learning value of the integral term) at the target convergence value or at the vicinity of the target convergence value when the control center air-fuel ratio is deviating from the target air-fuel ratio.
However, if external interferences with respect to the air-fuel ratio control, such as the cut-off of fuel supply and an increase in the fuel injection amount, frequently occur, the integral term (or the learning value of the integral term) may deviate from the target convergence value. For example, in the case where the fuel supply is cut off frequently, the air-fuel ratio in the catalyst is biased to the lean side and therefore the oxygen concentration sensor outputs a value corresponding a lean air-fuel ratio. This may cause a problem that the value of the integral term (or the learning value of the integral term) gradually deviates from the target convergence value to make the air-fuel ratio richer.